The present invention relates to recording systems and particularly to those systems employing continuous buried control signals such as used in servos for faithful track following operations to enable high track densities with high lineal densities.
It has long been recognized that, if a servo signal, i.e., a track location indicating signal, could be superposed with a data signal, higher areal density recording can be more easily achieved with a high degree of faithful reproduction. Even though such techniques have been known to be desirable, they have never been commercially exploited, apparently because of the degradation of the so-called buried servo or track-position indicating signals or the expense of providing a multiple layer magnetic coating, one for the position indicating signals and another for the data signals.
An early teaching of superposed data and servo signals is found in the F. J. Sordello U.S. Pat. No. 3,404,392. This patent teaches using a dual layer magnetic coating on a disk substrate. A lower layer has a higher retentivity and coercivity than an upper layer. A servo signal is first recorded in the lower layer for indicating track position. Following such recording, data signals were recorded. Then both the servo and data signals were sensed. There were no recording operations based upon sensing the servo signals. For each data recording operation, an erase gap preceded a read/write gap so that the data signals could be successfully recorded in the upper layer. During readback not only were the data signals read back by a single gap, but also two sets of servo signals were simultaneously read back. The Sordello servo signals were linearly recorded sine waves written in concentric circular tracks and exhibiting a low frequency. Adjacent servo tracks were written at different frequencies. Each data track was defined as being superposed over one-half of an odd numbered servo track and one-half of an even numbered servo track. Sordello teaches that the data signals and the servo signals must exist in the magnetic medium independently of the other. Sordello also teaches that in a dual layered record medium recording the data signals at a high frequency, at least three or four times the frequency of the servo signals in the lower layer, enables the data signals to be separated from the servo signals by filtering. The independence requirement was met by Sordello providing a record disk having a lower level with a higher coercivity than an upper data signal layer. This arrangement allowed the servo signals to remain undisturbed by subsequent writing and rewriting of data signals in the upper layer. The single gap in the Sordello recorder was significant for purposes of having perfect alignment for simultaneously reading the control or servo signals with the data signals.
A later development in the quest for achieving buried servo operation is shown by T. A. Schwarz in the IBM Technical Disclosure Bulletin, Vol. 17, No. 2, July 1974, beginning at page 536. Schwarz shows a single magnetic coating on a substrate. Schwarz teaches that the ratio of the frequency of the data signals to the servo signal should be 10:1 and that the servo signal can be successfully recorded with a head having sufficient ampere-turns capability. The Schwarz system apparently resulted in degradation of the recorded servo signals by repeated partial erasures each time data signals were recorded thereon. It also should be noted that Schwarz teaches it is necessary to erase before rerecording data signals. Schwarz also suggests the use of DC bias recording.
M. R. Cannon in U.S. Pat. No. 3,765,005 teaches using a clock-bias signal at the upper end of a data signal frequency spectrum. Also, U.S. Pat. No. 3,821,798 shows a low frequency control signal recorded at a lower end portion of the data signal pass band.
A later reference showing buried servo is Beecroft et al U.S. Pat. No. 3,956,769. Beecroft et al teach the use of two separate gaps in a transducer; one for servo signals and one for data signals. The present invention obviates the requirement for separate gaps.